Corrosion-resisting steel and method of processing



CORROSION-RESISTING STEEL AND METHOD OF PROCESSING 3 Sheets-Sheet 1 Filed April 10, 1965 lo l 6O (Hours) Time INVENTORS Remus A. Lulu, William G. Renshuv ATTORNEY May 10, 1966 R. A. LULA ETAL 3 Sheets-Sheet 2 Filed April 10, 1963 2: 9; 3 3.26 ow M cm I mp w 03 wk cow oom ON N OE oom T 1 w i m Ch I000 INVENTORS Remus A. Lulu, William G. Renshcw May 10, 1966 R. A. LULA ETAL 3,250,611

CORROSION-RESiSTING STEEL AND METHOD OF PROCESSING Filed April 10, 1963 3 Sheets-Sheet 3 cycles Toiol Weight Change x I000 IN V EN TORS Remus A. Lulu, William G. Renshow ATTORNEY United States Patent 3,250,611 CORROSION-RESISTING STEEL AND METHOD OF PROCESSING Remus A. Lula and William G. Renshaw, Natrona Heights, Pa., assignors to Allegheny Lndlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsyl- Vania Filed Apr. 10, 1963, Ser. No. 272,009 11 Claims. (Cl. 75-126) This is a continuation-in-part of application Serial No. 52,187, filed August 26, 1960, now abandoned.

This invention relates to a substantially completely ferritic, corrosion-resisting steel, and in particular to a nonhardening corrosion-resisting steel with improved ductility, outstanding deep drawing characteristics, excellent weldability and corrosion resistance in atmospheres containing internal combustion engine exhaust products and which is characterized by being extremely suitable for use in automotive exhaust systems.

Among the so-called stainless steels, the austenitic types are characterized by their excellent corrosion resistance, their case of fabrication reflected by their high ductility, weldability and non-hardening characteristics, and moderate levels of strength. The austenitic stainless steels include the AISI type 300 series as characterized by the 18% chromium-8% nickel steels, the AISI 200 series as characterized by the chromium-nickel-man-ganese steels and by the chromium-manganese-nitrogen steels. In contrast to the austenitic stainless steels, the straight chromium stainless steels are of two types, either the nonhardening or hardening stainless steels. Of this latter group, the hardenable straight chromium stainless steel-s are used for their strength and corrosion resistance; however, these steels exhibit poor fabrication characteristics, especially weldability. Of the straight chromium stainless steels of the non-hardening variety, the ferritic straight chromium steels, such as AISI type 430, have slightly better fabrication properties; however, their ductility and weldability are not adequate. The present invention is directed to a straight chromium, corrosion-resisting steel which exhibits excellent ductility and weldability.

An object of this invention is to provide a ferritic type steel having good corrosion resistance and excellent ductility and weldability. Y

Another object of this invention is to provide a fe-rritic, corrosion-resisting steel which is non-hardening and which may be readily worked to extremely thin gauges.

A more specific object of this invention is to provide a ferritic, corrosion-resisting steel essentially comp-rising carbon, silicon, manganese, chromium, titanium and iron, which steel has excellent weldability and excellent ductility.

Another more specific object of this invention is to provide a straight chromium, corrosion-resisting steel which is especially suitable for use in the exhaust systems of automobiles.

Other objects of this invention will become apparent 3,250,611 Patented May 10, 1966 FIG. 3 illustrates the effect of a varying titanium content in the steel of the present invention on its performsilicon, up to 1% maximum manganese, from 10.0% to 12.5% chromium, from 0.20% to 0.75 titanium and the balance iron with incidental impurities.

The carbon content is preferably kept low, and in no case should it exceed about 0.10%. With this amount of carbon present, the titanium content may be eilectively combined with the carbon in preference to the formation of chromium carbide, thus permitting the maximum amount of chromium to be available for corrosion resistance as will be more fully explained hereinafter. Also, by combining titanium with carbon, very little carbon enters into solution within the matrix, resulting in the low matrix carbon content contributing to the great ductility of the alloy and to its ferri tic character, thus permitting a cooling of the steel from heat treatment temperatures at any sufliciently fast rate without the danger of the formation of any of the martensite phase.

The chromium content of the steel of the present invention must be maintained within the range between 10.0% and about 12.5 Where the chromium content is decreased to amounts below about 10% the corrosion resistance is seriously adversely aflected. Moreover, in steels having .a lower chromium content, i.e., a chromium content of less than 10%, they are characterized by exhibiting poor ductility, especially in the welded condition. Chromium contents in excess of about 12.5% do not show any significant further increase in the corrosion resistance exhibited by the steel, especially where it is utilized in atmospheres containing the combustion products of an internal combustion engine, as for example, in exhaust mufiiers. Moreover, the higher amounts of chromium add materially to the cost of the alloy without producing significant changes in the mechanical or chem ical properties exhibited by the alloy.

The titanium content must be maintained within the range between about 0.20% and about 0.75 Titanium contents of less than about 20% do not tie up sufiicient carbon, with the result that the carbon is available for combining with the chromium, thereby depleting part of the matrix of its chromium content to adversely affect the corrosion resistance of the steel.- Moreover, if the steel is deficient in titanium, that'is, below about 0.20%, part of the carbon content may remain uncombined with the result that the uncombined carbon may be permitted to go into solution at the heat treating or welding temperatures, thereby producing a transformation product when the steel is rapidly cooled from the heat treating temperatures, with the efifect that the ductility of the steel, especially in the welded condition, is diminished with a corresponding increase noted in the yield strength and the tensile strength. Since the steel of the present invention is characterized by its excellent ductility, both in the unwelded and welded condition, it is necessary to have a minimum of 20% titanium within the steel of the present invention. On the other hand, where the titanium content is increased to beyond about 0.75%, excess titanium is found to be present with the result that the steel may show age hardening characteristics when exposed to elevated temperatures for prolonged periods of time, for example, when used as automobile mufliers.

In order to more clearly illustrate the composition of thesteel of this invention, reference is directed ,t Table Table I.Chemical composition (percent by weight) The steel of this invention may be made in any of the well-known manners employed in the production of nonhardening corrosion-resisting steels. Raw materials are melted, for example, in a carbon electrode electric arc furnace and are cast into ingots as is the usual practice. The pouring temperature is preferably above 2950 F., and extremely good results are obtained Where the pouring temperature is Within the range between 3050 F. and 3090 F. Good results have been obtained from ingots having a size varying between 19" x 35" up to 23" x 42" in cross sectional dimensions. The steel in the'form of ingots is thereafter soaked at a temperature of about 2350" F. and directly hot rolled to extremely light gauges, for example, to a hot rolled band having a thickness in the range between 0.070 and 0.275" without the necessity of reheating. n the other hand, if it is desired to produce a slab, the ingot may be soaked at a temperature of about 2350 F. and thereafter hot rolled to a slab. The slab is thereafter reheated to a temperature of about 2250 F. and hot rolled to the hot rolled band. A distinct advantage in the direct rolling of the steel of this invention from ingot to the hot rolled band is the fact that it eliminates the need for any intermediate conditioning since any discontinuities in the soundness of the metal product are readily sealed. The steel of the present invention is hot rolled with ease in comparison with type 430 which will be referred to hereinafter. Comparative tests indicated that it requires about 50% increase in power to hot roll type 430 steel than to roll the steel of the present invention to the same finished gauge of 0.1" from an entry gauge of 0.75. parative commercial processing of type 430 steel and the steel of the present invention, actual measurements indicate that with type 430 having an entry gauge of 0.75 and a temperature of 1800 F., it required 61 horsepower hours/ton to hot roll the steel to 0.10" with a finishing temperature of 1470 F., Whereas the steel of the present invention was hot rolled from the same entry gauge, at a temperature of 1780 F., to 0.10" and finishing temperature of 1440 F., yet the power consumption was only 41 horsepower hours/ton. Thus great savings areetfected in the hot rolling of the steel of the present invention.

Subsequent to the production of the hot rolled band, the steel of this invention may be directly cold rolled to finish gauge without the necessity of an intermediate annealing heat treatment. However, if the ultimate ductility is required in the steel of the present invention, it is preferred to anneal the steel at a temperature within the range between 1500 F. and 1700 F. and thereafter mechanically descale the steel prior to the cold rolling to the finished gauge. In any event, it is preferred to mechanically descale the steel prior to cold rolling, whether the steel is subjected'to an annealing heat treatment at this time or whether the steel is to be cold rolled without annealing heat treatment. While it is permissible to chemi- In com- 4 cally descale the steel after hot rolling, substantial savings are, effected by mechanically descaling the steel, for example by wheelabrating, prior to cold rolling. In either event, that is, whether the steel is mechanically or chemically descaled, the steel exhibits better corrosion resistance properties where it is descaled prior to cold rolling rather than cold rolling the steel directly from the hot rolling.

The steel of this invention "may be cold rolled, for example from 0.100" to 0.015", without the necessity for any intermediate anneal. It has been found that cold reduction of up to 85% may be effected to the steel of the present invention without the necessity of reannealing the steel. The extreme amount of cold rolling without the necessity of an intermediate anneal is possible because the, steel exhibits high ductility, low hardness and possesses a vary low rate of work hardening, thus enabling extremely large reductions. Similar to what was observed in hot rolling, the power requirements for effecting the same amount of cold reduction are far less for the steel of the present invention than for steel of type 430. The steel of the finished gauge is thereafter given a final annealing heat treatment and a final descaling operation. Preferably the steel is given an annealing heat treatnient of a temperature in the range between 1500 F. and about 1700 F., and excellent result have been obtained where the annealing heat treatment has been maintained within the temperature between the range of 1550 F. and 1600 F. Thereafter the steel is given a final descaling operation which is preferably a chemical pickle, and outstanding results have been obtained where the steel has been subjected to a pickling schedule which includes a solution of hot sulfuric acid or a 12% solution of sulfuric acid using an electrolytic pickling arrangement followed by a second pickling containing a solution of about 12% to 13% nitric acid and about 3% of hydrofluoric acid and a third pickling consisting of 7% nitric acid with a water rinse interposed after each pickling operation. The steel, after final annealing and final pickling operations, may thereafter be fabricated into the desired component, for example, a component of an automotive exhaust sys tern, and in particular an automobile muffler. Where desired, it is also possible to eliminate the final anneal and final pickling operations, and the steel may be used in its cold rolled condition.

In order to more clearly demonstrate the advantages of the steel of this invention, reference is directed to Table II which illustrates, in addition to a steel MF-l of this invention, the composition of a number of the straight chromium stainless steels which are of the characteristic 12% and 17% chromium varieties.

Table 11 Heat No. Type C Si Mn Cr Ti Fe 050 46 40 12. 77 Balance 052 31 27 12. 41 D0. 13 33 33 12. 35 Do. .13 .40 .44 12.43 Do. .052 36 .42 16. 25 Do. 075 .35 .46 16. Do. 058 49 00 16. 69 35 Do. .060 .63 .44 17. 24 58 D0. .040 63 49 17. 15 57 Do. 043 43 49 11. 17 52 Do.

Each of .the steels set forth in Table II is a commercial heat, and these steels were tested along with the MF-l steel of the present invention to illustrate their mechanical properties at room temperature in the annealed condition. These test results are set forth in Table III.

Table III.Mechanical properties HeatNo. Type Gage t ii $2585?! (Egg-t) Direcflm figs?- (p.s.i-) (a (Rb) 410 .0 9 47.980 71,110 5318 iiiiiliji 78 430 .036 51.030 68.770 33:8 iii iaiiii} 84 430 Ti 12,160 72,500 8 Transv.. 80

MF-l 1 34,850 64,340 i 3218 ii n elilii 71 MF-l .031 30,420 04,140 71 MF-l .037 30,550 03,730 71 MF-l 37,510 631680 Transvun 71 type 405 also contains about 0.20% aluminum. Accordingly, it would be expected that type 410 would have a slightly higher strength and hardness than the type 405.

When the chromium content is increased to about 17%,

for example in steels of type 430 and type 430 Ti, it will be noted that these steels possess substantially similar properties although the latter steels have a somewhat higher hardness. Type MF-l, a steel within the scope of the present invention, exhibits a yield strength of about 35,000 p.s.i. and a tensile strength of above 63,000 p.s.i. These mechanical properties are more than adequate, especially when these steels are used in exhaust systems of automobiles.

It is interesting to note that the steels of type MF-l show the highest ductility and the lowest hardness. Consequently, from the standpoint of mechanical properties, the steel of type MF-l has excellent fabrication properties. The ductility of this steel as measured in the standard tensile test as reported in Table III is excellent, and this ductility is confirmed when measured by the Erichsen and Olsen cup tests and is considerably better than the ductilities exhibited by all of the known ferritic stainless steels.

The steel of the present invention exhibits an outstanding measure of drawability and stretchability, especially as measured by the Swift test. This test utilizes a doubleacting press in which metal blanks are drawn using a round-nose punch of a given diameter in the formation of a cup which demonstrates both stretchability and drawability. The degree of drawability is measured as a percentage reduction and is computed on the basis of the original diameter of the metallic blank minus the diameter of the finished cup divided by the original diameter, the whole equation times 100. For purposes of comparison, it should be noted that austenitic steels of the A181 type 300 series stainless steels exhibit a percentage reduction of about 50%. As is well known, austenitic stainless steels exhibit a high degree of drawability and stretchability. Utilizing this same test, ferritic type stainless steels, for example AISI type 430 stainless steels, exhibit a percentage reduction which may be as high as 45%. In contrast thereto, the steel of the present invention has been drawn in the Swift test wherein the percentage reduction has exceeded 60%. Thus, as measured by the Swift test, the steel of the present invention exhibits outstanding drawability chacteri-stics which even exceed those of the austenitic stainless steels.

The weldability of the steel is similar to that of the austenitic stainless steels and shows considerable improvement over all of the known ferritic stainless steels. Substantially perfect ductility is achieved in the weld as illustrated by the bend tests and the high elongation in the tensile tests.

The weldability of corrosion-resisting steel, type MF -l, was evaluated by using an automatic tungsten inert gas and electrical resistance welding process. These two welding processes would most probably be used commercially to fabricate automotive mufilers from light gauge steels. But welds were made between pieces of type MF-l material by the automatic tungsten inert gas process without any filler metal additions. Both transversely and longitudinally welded tensile specimens were prepared from these welds, and bends were made of 180 around a IT diameter pin without cracking. The following Table IV illustrates the effect of welding on the mechanical properties of the steel.

Table IV 2% UTS, Percent Tensile properties R YS, p.s.i elongap.s.i tion Unwelded 66 30, 300 55, 34 Transversely welded 66 31, 400 50, 000 32. 5 Longitudinally welded 66 34, 400 57, 100 32 In order to more clearly demonstrate the effect of chromium on the steel of the present invention, reference is directed to Table V which contains the chemical analysis of five heats of substantially identical composition except for the chromium content. These five heats have chromium contents of 8.03%, 9.03%, 10.10%, 11.20% and 11.99%, respectively. These steels were welded using the automatic tungsten inert gas process without any filler metal additions.

Table V.-Cm position (percent by weight) Heat No. 0 Mn Si Cr Ni Ti Fe 063 44 43 8. 03 16 49 B211. .053 .46 .45 9. 03 18 .47 Bel. .053 .48 .45 10.10 .17 .50 B211. .034 .46 .43 11.20 .48 Bal. 04B 49 ll. 99 18 Bal.

Standard tensile specimens of the welded steels were tested in the longitudinal direction of the weld, utlizing the standard tensile test. Tabel VI includes the test results thereof.

Table VI.Tensile properties It is quite evident from the tensile test of the TIG welded specimens for Heat 942 that 8.03% chromium is inadequate in the steel of the present invention in order for the steel to possess excellent weldability characteris tics as manifested by the as welded tensile properties. The data in Table VI clearly shows that with a low chromium content, that is, below about 10% the ductility has decreased by more than one-half after welding. Thus it i clear that the chromium content of 8.03% is insufficient to confer sufficient ductility on the steel as welded. Heat 929 showed substantially similar results when the tensile properties in the Welded and unwelded conditions are compared. It is to be noted that while there has been some decrease in the 1% yield strength and ultimate tensile strength, between Heats 942 and 929 in the welded condition, yet the increase in the percent elongation is very slight compared with the ductility of Heat 929 in the unwelded condition. It is clear that the chromium content of 9.03% is inadequate in the steel of the present invention. In contrast thereto, Heat 930, which contains 10.10% chromium, exhibits substantially little change in the ductility in the welded condition compared to the unwelded condition. -Moreover, little change is manifested in the 2% yield strength and ultimate tensile strength. Where the chromium content is increased to 11.20%, as in Heat 931, it is also clear that very little change is noted in the .2% yield strength, ultimate tensile strength and ductility Heat 932 illustrates the effect of increasing the chromium content to about 11.99%. A comparison of Heat 931 and Heat 932 clearly illustrates that little change is ejected in both the unwelded and welded conditions. Thus, from the foregoing considerations alone it is clear that the/chromium content must be maintained at a minimum of about 10% since chromium contents below about 10% seriously adversely affect the ductility of the steel in the welded condition. Increasing the chromium content to more than about 12% does not have any significant effect on the mechanical properties of the steel in either the welded or unwelded condition.

In addition, electrical resistance spot welds were made in type MF-l material using the conditions recommended by the American Welding Society for 0.030 stainless steel. Weld time cycles of 4, 6, 8 and 10 produced tension shear strengths of 730, 810, 950 and 1030 lbs., respectively. These values are all greater than the minimum strength required by the American Welding Society for spot welds in steels having a thickness of .030.

, The most significant property of this steel resides in its corrosion resistance. In this respect, the corrosion reof this invention contains much lower chromium.

sistance of this steel is equal to and in many cases superior to that of type 410 stainless steel, even though the. steel Unlike type 410 ferritic stainless steel, the steel of the present invention has a low carbon content and, in addition, the steel of this invention contains titanium. With the titanium present within the amounts stated in Table I, the chromium is not tied up in the form of chromium carbides, but instead remains available to impart excellent corrosion resistance to this steel. The titanium has a greater afiinity for carbon than chromium, and as a result any excess carbon which is not in solution forms titanium carbide. With the low carbon levels that are employed in this steel, a slight excess of titanium is present, and thus improved corrosion resistance is obtained which is greater than that exhibited by type 410.

In order to more clearly demonstrate the corrosion resistance of the steel of this invention, reference is had to the following accelerated mufiier test. This test, which is employed by automotive muffler manufacturers, is designed to evaluate the performance of materials for use in automotive mufflers and consists of forming a solution containing 100 ml. of 1 N HBr solution, 100 ml. of 5 N H 50 with the balance distilled water to make up ten liters. Two liters of this solution are placed in a fiveliter battery jar and heated so that condensate is maintained at a temperature of about 185 F. Each test cyc e consists of immersing 2" X 4" specimens in the solution once every hour, and for the balance of the hour the specimens are suspended in the vapors of the test solution. This is continued for a period of seven hours, after which the specimens are placed in an oven at 475 F. for one hour. After cooling to room temperature, each of the specimens is weighed to obtain the Weight change per cycle. The eight-hour cycle is continued for 250 hours for a total of 31 cycles. Reference is directed to Table VII which illustrates the test results on a number of steels which have been subjected to the above-described accelerated muflier test.

Table VI] Material (type) Weight Loss (g) MF1 .055

Al coated carbon steel .18 Al dipped carbon steel 1 .20 Zinc dipped carbon steel 1.83 Type 405 .081 Type 410 .085 Type 430 .019

From Table VII it is clear that there is a more than 50% increase in weight change of type 405 and type 410 over the steel of the present invention. When compared with the aluminum coated carbon steel or aluminum dipped carbon steel, it will be seen that the latter exhibits an increase in weight change of about 350% to 400% more than the steel of the present invention. Thus, it is clear that the steel of the present invention exhibits outstanding corrosion resistance properties.

In order to more clearly show the outstanding corrosion resistance of the steel of the present invention in comparison with other ferritic non-hardening stainless steels, as well as some other known and presently used mufiier steels, reference is bad to the accompanying drawing. The curves of the drawing illustrate the relation between the weight loss and the testing time of the accelerated muffler test as described hereinbefore. Curve 10 illustrates the performance of aluminum dipped carbon steel, a well-known material used in automotive mufiiers. It is clear that there is an excessive weight loss after a very short period of time. This weight loss appears to increase linearly with time for the period of the 250 hours tested. Substantially similar results are shown for aluminum coated carbon steel as illustrated by curve 20. Curve 30 illustrates the performance of type 410 stainless steel,

whereas curve 40 illustrates the performance of type 405 stainless steel, the chemical compositions of which show an increase of about 1% in the chromium content over that used in the alloy of the present invention. From curves 30 and 40 it is clear that there is little difference in behavior between the two; however, each is far superior to aluminum protected carbon steel. Curve 50 illustrates the performance of the steel of the present invention. As is clear from the figure, the steel of the present invention is superior to types 405 and 410 on the basis of this test. Curve 60 illustrates the performance of steel of the type 430 composition. As would be expected, the steel of the type 430 composition performs better than the other steels tested, including the steel of the present invention; how ever, it will be appreciated that the steel of the 430 type contains 17% chromium, whereas the steel of the present invention contains nominally about 11% chromium.

Chromium is quite critical in the steel of the present invention, especially from the standpoint of the accelerated mufller corrosion test, since the steel of the present invention finds an important usage in the exhaust components of an internal combustion engine, for example a muffler. In this respect it has been found that the chromium content must be maintained within the limits set forth hereinbefore in Table I. Reference is directed to FIG. 2 which illustrates the effect of chromium on the performance of the steel in the accelerated mufller corrosion test. FIG. 2 is a plot of the total cumulative weight change versus the number of cycles for the accelerated muflier corrosion test which was described hereinbefore.

In FIG. 2, curve 70 illustrates the performance of a steel in the muffler corrosion test which has a chromium content of 8.03%. This is Heat No. 942 which has a composition as set forth in Table V. Curve 72 shows the performance of a steel having 9.03% chromium, and is the composition of Heat No. 929 of Table V. It is quite evident that the steels of curves 70 and 72 are totally inadequate from the standpoint of their corrosion resistance, as illustrated by the rapid weight change. However, where the chromium content is increased to more than about as for example with the steel of curve 74 which is Heat No. 930 having the composition set forth in Table V, it is immediately seen that an outstanding improvement is effected to the steel in its performance in the accelerated muffler corrosion test. While it is preferred to have a higher chromium concentration, that is, about 11% or more as illustrated by the curve '76, which is for Heat No. 943 having the composition of 0.07% carbon, 0.44% manganese, 0.44% silicon, 11.10% chromium, 0.17% nickel, 0.50% titanium and the balance iron, none the less it is quite clear that at least 10% chromium must be present. Where the chromium content is increased to about 12%, as for example in the steel of Heat No. 932 shown in curve 78, substantially little differsignificance is curve 80 which illustrates the performance of an AISI type 430 stainless steel having a nominal chromium content of about 16.5%. It is to be noted that while there is an additional 4.5% chromium over and above that set forth for the steel of curve 78, it is clear that there is very little change in the slope of curve 80. Consequently, it is preferred to maintain the upper limit of the chromium content at about 12.5%. These curves clearly demonstrate the necessity for the limitations placed upon the chromium content, especially from the standpoint of the corrosion resistance as illustrated in the accelerated mufiler corrosion test.

The effect of titanium can also be demonstrated in this same mufiier test. Reference is respectfully directed to FIG. 3 which contains the same plot as that of FIG. 2, but with various changes in the titanium composition. Curve 90 illustrates the performance of a typical AISI type 410 stainless steel which has no deliberate additions of titanium thereto and contains a nominal chromium content of about 12.5 Despite a high chromium content as compared to Heat 943, it is clear that Heat 943 possesses superior corrosion resistance. The addition of about 0.25% aluminum to the same basic composition of A181 type 410 is illustrated by curve 92 which is a curve of anAISI type 405 stainless steel which contains nominally 12.5% chromium and 0.25% aluminum. It is to be noted that both type 410 and type 405 show some divergence as the tests continue; however, there is substantially little difference in the slope of these curves. Their relative placement is much higher, however, demonstrating their poorer corrosion resistance. In contrast thereto, curve 94 illustrates the effect of 0.20% titanium on a steel having chromium content of 11.30%. This steel is Heat No. 926 having a composition, 0.029% carbon, 0.96% manganese, 0.42% silicon, 11.30% chromium, 0.15% nickel, 0.20% titanium and the balance iron. It is apparent that the steel of curve 94 has a far superior corrosion resistance in the accelerated muffier corrosion tests than that exhibited by either type 405 or type 410 stainless steel. This is so, despite the fact that the steel of curve 94 contains about 1.25% less chromium but, in addition, also contains .20% titanium. Moreover, it is abundantly clear that the aluminum cannot be substituted for the titanium on an equal basis and still obtain the same degree of corrosion resistance. Where the titanium content is increased to 0.48%, as in Heat 931 having the composition set forth in Table V and curve 96, it is clear that the higher amounts of titanium are accordingly preferred. In this respect it is to be noted that both the steel of curve 94 and the steel of curve 96 possess outstanding corrosion resistance characteristics as measured by the accelerated mufiler corrosion test. On

' the other hand, where the titanium content is increased to beyond the upper limit, as set forth hereinbefore, it is clear that the initial corrosion resistance of this steel is very poor. This is clearly demonstrated by curve 98 of FIG. 3 for Heat 927 having the composition 0.028% carbon, 0.44% manganese, 0.45% silicon, 10.88% chromium, 0.15% nickel, 0.97% titanium and the balance iron. While the corrosion resistance improves With time, it is clearly indicative of the characteristic of this steel which has an excess amount of titanium and consequently is usually quite dirty from a quality control standpoint. The source of the dirt is usually inclusions which clearly accelerate the corrosion of this steel. Accordingly, it is clear that the titanium content must be maintained Within the range set forth hereinbefore in Table I.

The steel of the present invention also compares favorably when tested using the standard salt spray test which was run in accordance with ASTM specifications. The steel of the present invention proved to be superior to type 410 as illustrated by the test results set forth in Table VIII below.

Table VIII Material: Results MF-l Six rust spots with bleeding. Type 410 Covered with rust spots and bleed- Al coated ing.

carbon steel No rusting-covered with white Al dipped deposit.

carbon steel N0 rusting-covered with white deposit. Carbon steel Covered completely with rust.

While Table VIII illustrates that the aluminum coated or dipped carbon steel showed no rusting, it would be expected that once the aluminum coating is pierced accelerated rusting of the carbon steel would ensue. As indicated by the white deposit on the aluminum coated or dipped steels, the aluminum is being attacked by the salt spray, and the flaking of this deposit will eventually expose the carbon steel to the salt spray and rusting will be initiated after an extremely short exposure time.

The steelof this invention requires no special skills or equipment in the handling, manufacture or fabrication thereof. The steel possesses an outstanding combination of good fabrication properties, especially ductility and Weldability, as well as corrosion resistance, especially when used in automotive exhaust systems.

We claim:

1. A corrosion-resisting steel consisting essentially of, from 0.01% to 0.10% carbon, up to 0.75% silicon, up to about 1.0% manganese, from 10.0% to 12.5% chromium, from 0.20% to 0.75% titanium, and the balance essentially iron with incidental impurities, the steel being characterized by exhibiting a completely ferritic microstructure, high ductility, good corrosion resistance and excellent weldability.

2. A corrosion-resisting steel consisting essentially of, from 0.03% to 0.06% carbon, up to 0.30% silicon, up to about 0.5% manganese, from 10.5% to 11.5% chromium, from 0.30% to 0.50% titanium, and the balance essentially iron with incidental impurities, the steel being characterized by exhibiting a completely territic microstructure, high ductility, good corrosion resistance and excellent weldability.

3. A corrosion-resisting steel exhaust mufiler formed from a steel consisting essentially of from 0.01% to 0.10% carbon, up to 0.75 silicon, up to 1.0% manganese, from 10.0% to 12.5% chromium, from 0.20% to 0.75% titanium, and the balance essentially iron with incidental impurities.

4. A corrosion-resisting steel exhaust muffler formed from a steel consisting essentially of from 0.03% to 0.06% carbon, up to 0.30% silicon, up to 0.5% manganese, from 10.5% to 11.5% chromium, from 0.30% to 0.50% titanium, and the balance essentially iron with incidental impurities.

5. An article of manufacture particularly suited for use in an atmosphere containing combustion products of an internal combustion engine and characterized by having a ductility as measured by the percentage of elongation in two inches of at least 30%, said article being formed from a ferritic corrosion resisting steel consisting essentially of from 0.01% to 0.10% carbon, up to 0.75% silicon, up to 1.0% manganese, from 10.0% to 12.5% chromium from 0.20% to 0.75% titanium, and the balance essentially iron with incidental impurities,

6. An article of manufacture particularly suited for use in an atmosphere containing combustion products of an internal combustion engine and characterized by having a ductility as measured by the percentage of elongation in two inches of at least 30%, said article being formed from a ferritic corrosion-resisting steel consisting essentially of from 0.03% to 0.06% carbon, up to 0.30% silicon, up to 0.5% manganese, from 10.5% to 11.5% chromium, from 0.30% to 0.50% titanium, and the balance essentially iron with incidental impurities.

7. A corrosion-resisting steel consisting essentially of about 0.04% carbon, about 0.43% silicon, about 0.49% manganese, about 11.17% chromium about 0.52% titanium, and the balance essentially iron with incidental impurities.

8. In the method of producing corrosion-resisting steel which is particularly suitable for use in automotive exhaust systems, comprising, the steps of, making a melt of steel having a composition consisting essentially of from 0.01% to 0.10% carbon, up to 0.75% silicon, up to 1.0% manganese, from 10.0% to 12.5% chromium, from 0.20% to 0.75% titanium, and the balance essentially iron with incidental impurities, casting the melt into ingots, soaking the ingots at a temperature in the rangebetween 2250 F. and the incipient melting temperature, directly hot rolling the heated ingot to a hot rolled strip having a thickness in the range between 0.070 and 0.130" without any intermediate reheating or conditioning, descaling the hot rolled strip, cold roll reducing the descaled strip, and thereafter subjecting the cold roll reduced strip to a final annealing and pickling.

9. In the method of producing corrosion-resisting steel which is particularly suitable for use in automotive exhaust systems, comprising, the steps of, making a melt of steel having a composition. consisting essentially of from 0.01% to 0.10% carbon, up to 075% silicon, upto 1.0% manganese, from 10.0% to 12.5% chromium, from 0.20% to 0.75% titanium, and the balance essentially iron with incidental impurities, casting the melt into ingots, soaking the ingots at a temperature in the range between 2250 F. and the incipient melting temperature, directly hot rolling the heated ingot to a hot rolled strip having a thickness in the range between 0.070 and 0.130" without any intermediate reheating or conditioning, descaling the hot rolled strip, cold roll reducing the descaled strip to effect a reduction in the cross sectional area of up to 85% without any intermediate heat treatment, and thereafter subjecting the cold roll reduced strip to a final annealing and pickling.

10. In the method of producing corrosion-resisting steel which is particularly suitable for use in automotive exhaust systems, comprising, the steps of, making a melt of steel having a composition consisting essentially of from 0.01% to 0.10% carbon, up to 0.75% silicon, up to 1.0% manganese, from 10.0% to 12.5% chromium, from 0.20% to 0.75 titanium, and the balance essentially iron with incidental impurities, casting the melt into ingots, soaking the ingots at a temperature in the range between 2250 F. and the incipient melting temperature,

directly hot rolling the heated ingot to a hot rolled strip without any intermediate reheating or conditioning, de scaling the hot rolled strip having a thickness in the range between 0.070 and 0.130, cold roll reducing the descaled strip to effect a reduction in the cross sectional area of up to 85 Without any intermediate heat treatment, annealing the cold roll reduced strip at a temperature in the range between 1500 F. and 1700 F., and thereafter pickling the annealed strip.

11. In the method of producing corrosion-resisting steel which is particularly suitable for use in automotive exhaust systems, comprising the steps of making a melt of steel having a composition consisting essentially of from 0.01% to 0.10% carbon, up to 0.75% silicon, up to 1.0% manganese, from 10.0% to 12.5 chromium, from 0.20% to 0.75% titanium, and the balance essentially iron with incidental impurities, casting the melt into ingots, soaking the ingots at a temperature in the range between 2250 F. and the incipient melting temperature,

directly hot rolling the ingot to a hot rolled semi-inished mill product having a thickness in the range between 0.070" and 0.130" without any intermediate reheating or conditioning, descaling the hot rolled semi-finished mill product, and cold roll reducing the descaled semifinished mill product to effect a reduction in the cross sectional area of up to 85% without any intermediate heat treatment.

I References @ited by the Examiner UNITED STATES PATENTS 2,024,561 12/1935 Becket et al 126 XR 2,118,693 5/1938 Arness 148-135 XR 2,139,538 12/1938 Becket al '75126 XR 2,772,992 12/1956 Kiefer et al 148-2 2,905,577 9/1959 Harris et al 75126 XR OTHER REFERENCES Kinzel and Franks: Alloys of Iron and Chromium, vol. 11, page 54, published by McGraw-Hill Book Co., New York, N.Y.

DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. A CORROSION-RESISTING STEEL CONSISTING ESSENTIALLY OF, FROM 0.01% TO 0.10% CARBON, UP TO 0.75% SILICON, UP TO ABOUT 1.0% MANGANESE, FROM 10.0% TO 12.5% CHROMIUM, FROM 0.20% TO 0.75% TITANIUM, AND THE BALANCE ESSENTIALLY IRON WITH INCIDENTAL IMPURITIES, THE STEEL BEING CHARACTERIZED BY EXHIBITING A COMPLETELY FERRITIC MICROSTRUCTURE, HIGH DUCTILITY, GOOD CORROSION RESISTANCE AND EXCELLENT WELDABILITY. 